1. Field of the Invention
This invention relates to the testing of underground formations or reservoirs. More particularly, this invention relates to a method and apparatus for isolating a downhole reservoir, and testing the reservoir formation and fluid.
2. Background Information
While drilling a well for commercial development of hydrocarbon reserves, numerous subterranean reservoirs and formations will be encountered. In order to discover information about the formations, such as whether the reservoirs contain hydrocarbons, logging devices have been incorporated into drill strings to evaluate several characteristics of the these reservoirs. Measurement while drilling systems (hereinafter MWD) have been developed which contain resistivity and nuclear logging devices which can constantly monitor some of these characteristics while drilling is being performed. The MWD systems can generate data which includes hydrocarbon presence, saturation levels, and porosity data. Moreover, telemetry systems have been developed for use with the MWD systems, to transmit the data to the surface. A common telemetry method is the mud-pulsed system, an example of which is found in U.S. Pat. No. 4,733,233. An advantage of an MWD system is the real time analysis of the subterranean reservoirs for further commercial exploitation.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling, using the MWD systems, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested by means of other test equipment.
One type of post-drilling test involves producing fluid from the reservoir, collecting samples, shutting-in the well and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different reservoirs within a given well bore. This type of test is known as a Pressure Build-up Test. One of the important aspects of the data collected during such a test is the pressure build-up information gathered after drawing the pressure down. From this data, information can be derived as to permeability, and size of the reservoir. Further, actual samples of the reservoir fluid must be obtained, and these samples must be tested to gather Pressure-Volume-Temperature data relevant to the reservoir's hydrocarbon distribution.
In order to perform these important tests, it is currently necessary to retrieve the drill string from the well bore. Thereafter, a different tool, designed for the testing, is run into the well bore. A wireline is often used to lower the test tool into the well bore. The test tool sometimes utilizes packers for isolating the reservoir. Numerous communication devices have been designed which provide for manipulation of the test assembly, or alternatively, provide for data transmission from the test assembly. Some of those designs include signaling from the surface of the Earth with pressure pulses, through the fluid in the well bore, to or from a down hole microprocessor located within, or associated with the test assembly. Alternatively, a wire line can be lowered from the surface, into a landing receptacle located within a test assembly, establishing electrical signal communication between the surface and the test assembly. Regardless of the type of test equipment currently used, and regardless of the type of communication system used, the amount of time and money required for retrieving the drill string and running a second test rig into the hole is significant. Further, if the hole is highly deviated, a wire line can not be used to perform the testing, because the test tool may not enter the hole deep enough to reach the desired formation.
There is also another type of problem, related to down hole pressure conditions, which can occur during drilling. The density of the drilling fluid is calculated to achieve maximum drilling efficiency while maintaining safety, and the density is dependent upon the desired relationship between the weight of the drilling mud column and the downhole pressures which will be encountered. As different formations are penetrated during drilling, the downhole pressures can change significantly. With currently available equipment, there is no way to accurately sense the formation pressure as the drill bit penetrates the formation. The formation pressure could be lower than expected, allowing the lowering of mud density, or the formation pressure could be higher than expected, possibly even resulting in a pressure kick. Consequently, since this information is not easily available to the operator, the drilling mud may be maintained at too high or too low a density for maximum efficiency and maximum safety.
Therefore, there is a need for a method and apparatus that will allow for the pressure testing and fluid sampling of potential hydrocarbon reservoirs as soon as the bore hole has been drilled into the reservoir, without removal of the drill string. Further, there is a need for a method and apparatus that will allow for adjusting drilling fluid density in response to changes in downhole pressures, to achieve maximum drilling efficiency. Finally, there is a need for a method and apparatus that will allow for blow out prevention downhole, to promote drilling safety.